Method and apparatus for an aircraft inner loop elevator control system

ABSTRACT

An improved inner loop control is disclosed for use in an aircraft pitch axis control system. Signals representative of vertical acceleration and pitch rate are complementary filtered and summed to produce an &#34;inner loop&#34; damping signal which is combined with &#34;outer loop&#34; command signals to produce an elevator control signal. The improved approach provides higher outer loop control law gains and better system stability, resulting in higher control bandwidth and tighter command tracking, especially in turbulence.

BACKGROUND OF THE INVENTION

The present invention pertains to the aircraft guidance art and, moreparticularly, to a system for controlling aircraft flight during landingflare.

A critical portion of aircraft landing trajectory is commonly known asflare-out or flare. Flare is that portion of the landing trajectorybetween the fixed angle glideslope and aircraft runway touchdown. Thus,it is desirable, particularly for commercial aircraft that the flareprofile depart smoothly from the fixed angle glideslope approach therebyproviding a smooth transition to runway taxiing.

Aircraft automatic landing flare performance is determined by a flarecontrol law. In the prior art, such control laws have not exhibitedsatisfactory stability and precision command tracking. As a result, theactual touchdown point of the aircraft on the runway has variedconsiderably. This is undesirable both for safety reasons and becausethere are tightening regulations on aircraft landing dispersion.

Thus, there has been a long felt need in the aircraft guidance controlart for improved aircraft flare control.

SUMMARY OF THE INVENTION

It is an object of this invention, therefore, to provide an improvedapparatus and methods for producing an aircraft inner loop elevatorcontrol signal.

It is a particular object of the invention to provide an improvedaircraft inner loop elevator control system which allows higher outerloop control law gains and better system stability and results in highercontrol bandwidths with tighter command tracking, especially inturbulence.

Briefly, according to the invention, improved aircraft control innerloop damping signals are developed and combined with outer loop commandsignals to produce an elevator control signal. The improved inner loopapparatus comprises means for producing an aircraft verticalacceleration signal h, means for producing an aircraft pitch rate signalθ, a low pass filter to attenuate the vertical acceleration signal habove a predetermined frequency ω, a high pass filter to attenuate thepitch rate signal θ below said predetermined frequency ω, and a summermeans that sums the filtered h and θ signals to thereby produce theimproved inner loop damping signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical prior art aircraft flare-out controlsystem;

FIG. 2 illustrates a basic flare trajectory law to track a flaretrajectory specified by the commands h_(c), h_(c) and h_(c) ;

FIGS. 3 and 4 are graphs comparing the frequency responsecharacteristics of the systems shown in FIGS. 1 and 2;

FIG. 5 illustrates the response characteristics of the systems of FIGS.1 and 2;

FIG. 6 is a block diagram illustrating one implementation of the h/θinner loop elevator control system;

FIGS. 7 and 8 are block diagrams illustrating implementation of thesystem of FIG. 6 without the requirement of an independent pitchattitude signal;

FIG. 9 is a block diagram illustrating implementation of a θ/θ innerloop control system which for one specific combination of gains exhibitsstability characteristics identical to the system of FIG. 2;

FIG. 10 is a graph illustrating the frequency responses of altitudedeviation due to horizontal gust for the control law design shown inFIGS. 2 and 9;

FIG. 11 is a block diagram of a control system which utilizes an h/θinner loop control in the low frequency range and the θ/θ inner loopcontrol in the high frequency range;

FIG. 12 is a block diagram illustrating the system of FIG. 11 reduced toa simplified form;

FIG. 13 is a block diagram showing the θ/θ control law system of FIG. 9with the addition of a washout circuit on the θ and θ feedback signals;

FIG. 14 is a block diagram of the complemented form of the system shownin FIG. 13;

FIG. 15 is a block diagram of a system which couples together thesystems of FIGS. 2 and 9 by means of a second order complementaryfilter;

FIG. 16 is a graph illustrating a response of the systems shown in FIGS.15 and 17;

FIG. 17 is a block diagram of a system derived from combining all liketerms in the system of FIG. 15; and

FIG. 18 is a variant of the system shown in FIG. 12 wherein the h signalis produced at the output of the complementary filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

During the course of comprehensive study to improve the automaticlanding flare performance of aircraft, various means were investigatedto increase the control bandwidth and improve the frequency response toturbulence of various flare control law configurations.

In these investigations, it was found that substantial improvementscould be made if the control law gains, in a base line design using aninner loop with vertical acceleration and pitch rate feedback, could beraised. FIG. 1 illustrates a base line design. A detailed description ofthe operation of the base line design shown in FIG. 1 will not bepresented here since it would be readily understood by anyone ofordinary skill in the art. Four signals are input to the system: h_(CF),representing sink rate derived by complementary filtering from h andh_(baro), h_(R) representing radio altitude, h representing verticalacceleration and θ representing pitch rate. The signals are processed,as shown, through summing circuits, such as summer 12, gain blocks, suchas block 14, filter blocks, such as blocks 16, 18 and logic block 20.

The glideslope signal δ_(e).sbsb.c is produced by a glideslope controlblock 22, such apparatus being well known in the prior art. This signalis coupled through to the output elevator command line 24 until suchtime as the various switches are activated, thereby initiating theaircraft flare. During flare-out, the signals received at the system'sinput are processed through the circuitry shown to produce the elevatorcommand signal at output line 24.

FIG. 2 illustrates a flare trajectory tracking law allowing gains higherthan the system of FIG. 1. As is shown in FIGS. 3 and 4, the altitudetracking error Δh of the control law of FIG. 2 due to gust issubstantially lower and the control bandwidth is substantiallyincreased. However, when the design of FIG. 2 was tested in thesimulator with detailed airplane and sensor characteristics, it wasfound that this system suffered from low damping and high elevatoractivity in turbulence. These problems were traced to the verticalacceleration (h) and pitch rate (θ) feedback inner loop. In particular,the vertical acceleration signal included signal components attributableto the location of the sensor relative to the airplane's center ofgravity. Further, the vertical acceleration signal was filtered toreduce high frequency noise components. The combination of these twofactors resulted in poor inner loop signal quality to damp the highfrequency pitch mode. It was found that the sensitivity to the aboveaffects had become much greater by the increase of the overall controllaw gains. Also, the high level of vertical acceleration gain increasedthe elevator activity to an unacceptable level (see FIG. 5). However,FIG. 5 shows that the altitude tracking of the control law of FIG. 2 isconsiderably improved over the base line system.

These deficiencies were overcome, while the higher gains and theassociated performance benefit were retained by elimination of the hsignal in the higher frequency range and substitution of pitch attitudeinformation which provides more effective damping information in thehigher frequency range.

FIG. 6 illustrates one manner of accomplishing this goal. Here, thevertical acceleration signal h is rolled off at frequencies ω>1/τ by thelag circuit 30 after being amplified by a gain factor k_(h) in gainblock 32. A signal θ representative of aircraft pitch attitude isbrought in at frequencies ω>1/τ by the washout circuit 34 which ispreceded by a gain block 36. Thus, the two signals are summed in summer40 and passed to a summer 42 where it is combined with conventionalouter loop command signals from block 44 and the pitch rate signal θ asamplified in gain block 46. The resultant output from summer 42 is theelevator control signal δ_(e).sbsb.c.

The inner loop configuration of FIG. 6 may be reduced to an alternateform by rearranging the pitch and pitch rate terms, and consideringsθ=θ. Thus, ##EQU1##

Implementation of the control law can, therefore be accomplished withoutthe need for an additional pitch attitude signal, as is shown in FIG. 7.Alternatively, the h and θ filters of FIG. 7 may be implemented as shownin FIG. 8.

The θ or lagged θ signal does not need additional noise filtering, isnot sensitive to sensor location and is an inherently more suitabledamping signal for the high frequency pitch modes. This "complementarylagged h/θ signal processing" eliminates the discussed problem inachieving a well damped high gain control law. It substantiallymaintains the superior turbulence response characteristics of a pure h/θinner loop system in the lower frequency range and improves the responsein the high frequency range due to the elimination of ineffectiveelevator activity.

A more sophisticated way of replacing the high frequency h signalcomponent with pitch attitude information was developed as follows.Using the linear equations of motion, the vertical acceleration signalcan be expressed as a linear function of state variables governing theairplane dynamics. Thus,

    h=K.sub.1 h+K.sub.2 h+K.sub.3 θ+K.sub.4 θ+K.sub.5 ΔV+K.sub.6 δ.sub.e +K.sub.7 ΔT+K.sub.8 u.sub.g +K.sub.9 W.sub.g                                                   (2)

In this equation, K₁ -K₉ are constants, h represents vertical position,h vertical velocity, θ and θ represent pitch and pitch rate,respectively, ΔV represents aircraft deviation from the reference speed,δ_(e) represents elevator deflection, δT represents thrust change, U_(g)represents horizontal gust velocity and W_(g) represents vertical gustvelocity. Also, the angle of attack (α) terms have been expressed as afunction of θ and h. This equation, with the δT, U_(g) and W_(g) termsneglected, was used to replace the vertical acceleration feedback signalin FIG. 2 with its equivalent signal components. The result is the θcontrol law of FIG. 9. This system has stability and elevator responsecharacteristics identical to the control law of FIG. 2, although all thegains are different. It was found that the ΔV term has little effect onstability and can therefore be eliminated. The turbulence response ofthe two control laws are entirely different, since the gust terms wereomitted in the h replacement. It is not surprising that the gustresponse of the control law of FIG. 9, with K.sub.ΔV is generally poorerthan for the control law of FIG. 2, as is illustrated in FIG. 10.However, in the frequency range above 1 radian per second the θ/θcontrol law of FIG. 9 with K.sub.ΔV =0 has the best response.

An overall superior control law can therefore be synthesized using theh/θ control law of FIG. 2 for the low frequency range and the θ/θcontrol law of FIG. 9 for the high frequency range. These two controllaws can be coupled together to work in their respective best frequencyranges, again using complementary filtering techniques. The resultantsystem is illustrated in FIG. 11.

The system of FIG. 11 can be reduced to a simplified form shown in FIG.12. This control law was evaluated in a simulation and demonstrated verygood performance characteristics.

Further improvements were found possible. The gust response of the θ/θcontrol law of FIG. 9 was improved by placing a washout circuit on the θand θ feedback signals. The resulting system is shown in FIG. 13. Thisfrees the pitching motion in the low frequency range and thus helps toalleviate gust loads and keep the airplane on its path.

Incorporating complementation, the resulting system is shown in FIG. 14.For this control law full performance evaluation was conducted on asimulator and in flight tests. The results showed an approximately 50percent reduction in touchdown dispersion over the base line system.

The control laws of FIGS. 2 and 9 can be coupled together by means of asecond order complementary filter. The result is shown in FIG. 15. Thisproduces yet a better adherence to the h/θ control law frequencyresponse for low frequencies and the θ/θ control law frequency responsefor the high frequencies, as is shown in FIG. 16. After combining alllike terms in FIG. 15, the control law of FIG. 15 reduces to the formshown in FIG. 17. Besides high gain (bandwidth), good stability, thebest gust response, low elevator activity and insensitivity to h signalanomalies, this control law features an inherent washout on the innerloop signals h and θ, thereby making the control law immune to static orlow frequency inner loop sensor errors. This is of special importance inautoland control law design, where sensor bias errors can otherwisecause tracking offsets resulting in unacceptable autoland performance.

Additional, important features realized with the control laws of FIGS.8, 12, 14 and 17 are implicit h signal derivation and complete controllaw initialization, thereby eliminating pre-flare terrain effects.

The implicit h signal derivation in the control law of FIG. 17 isevident; no h signal feedback appears in the control law. This isespecially attractive because it is difficult to obtain an accurate hsignal.

The h and θ lag filters are initialized at zero, with the h lag filterbeing initialized to carry the last computed elevator command from theprevious control mode. The washout filters need not be initialized.Since none of the filters carry pre-flare signal information, pre-flaresignal characteristics such as are exhibited by radar altitude cannoteffect the flare.

The control law of FIG. 12 will be used to show how the complementarywashout/lag filter may be employed to achieve the same result.

A sink rate signal with desired characteristics can be synthesized fromradio altitude h_(R) and vertical acceleration from an inertialreference system (h_(IRS)) as: ##EQU2##

Both the lagged h_(IRS) and the washed out h_(R) signal components arealready developed in the washout/lag complementary filter shown in FIGS.12 and 14. The h signal can therefore be produced on the output of thiscomplementary filter with the desired gain simply by increasing the hand h input gains in this filter. This is shown for the control law ofFIG. 12 in FIG. 18. A typical filter time constant of one second is usedwhich is suitable for both the h signal derivation and for the h/θcomplementation.

Since, in the control laws of FIGS. 8, 12 and 14, the complementaryfilter is the only one requiring initialization, it may be used to carrythe last command from the previous control mode as the initialcondition, thereby again avoiding pre-flare sensor history effects.

In summary, an improved pitch control inner loop feedback system hasbeen described which allows higher control law gains with good systemstability. The system realizes improved path tracking and turbulence,lower pitching and elevator activity and improved touchdown dispersions.

These results are achieved through a complementary h/θ filteringtechnique. This technique further provides implicit sink rate signalderivation and control law initialization preventing pre-flare signalhistories (e.g. terrain effects on radio altitude) from affecting theflare control.

A final control law is described having, besides the above features, theadded characteristic of washing out all inner loop feedback signals,thereby further reducing the effects of low frequency sensor errors onthe landing performance.

While preferred embodiments of the invention have been described indetail, it should be apparent that many modifications and variationsthereto are possible, all of which fall within the true spirit and scopeof the invention.

We claim:
 1. In an aircraft flight control system wherein inner loopdamping signals are combined with outer loop command signals to producean elevator control signal, improved inner loop damping signal apparatuscomprising:means for providing a signal h representative of aircraftvertical acceleration; means for amplifying said h signal by apredetermined gain factor K_(h) ; means for providing a signal θrepresentative of aircraft pitch; means for amplifying said θ signal bya predetermined gain factor K.sub.θ ; low pass filter means forattenuating said h signal above a predetermined frequency ω; high passfilter means for attenuating said θ signal below said frequency ω; meansfor providing a signal θ representative of aircraft pitch rate; meansfor amplifying said θ signal by a predetermined gain factor K.sub.θ ;and summer means for summing said low pass filtered h signal, said highpass filtered θ signal, said amplified θ signals and the outer loopcommand signal to thereby produce the control signal.
 2. In an aircraftflight control system wherein inner loop damping signals are combinedwith outer loop command signals to produce an elevator control signal,improved inner loop damping signal apparatus comprising:means forproviding an aircraft vertical acceleration signal h; means forproviding an aircraft pitch rate signal θ; means for amplifying said hand θ signals by predetermined gain factors; first low pass filter meansfor attenuating said vertical acceleration signal h above apredetermined frequency ω; high pass filter means for attenuating saidpitch rate signal θ below said predetermined frequency ω; second lowpass filter means for attenuating said pitch rate signal θ above saidpredetermined frequency ω; and summer means for summing said low passfiltered h and said high pass and low pass filtered θ signals to producean inner loop damping signal.
 3. In an aircarft flight control systemwherein an inner loop command signal is combined with an outer loopdamping signal to produce an elevator control signal, an improved methodof producing said inner loop damping signal comprising the steps of:(a)providing an aircraft vertical acceleration signal h; (b) providing anaircraft pitch rate signal θ; (c) amplifying said h and θ signals bypredetermined gain factors; (d) attenuating said vertical accelerationsignal h above a predetermined frequency ω; (e) attenuating said pitchrate signal θ below said predetermined frequency ω; (f) attenuating saidpitch rate signal above said predetermined frequency ω; and (g) summingsaid attenuated h and θ signals from steps (d), (e) and (f) to producethe inner loop signal.
 4. In an aircraft flight control system,apparatus for producing an elevator control signal, the apparatuscomprising:means for providing signals θ and θ representative ofaircraft pitch and pitch rate, respectively; means for providing signalsh, h and h representative of aircraft altitude, altitude rate andvertical acceleration, respectively; first high pass filtering means forattenuating said θ signal below a predetermined frequency; firstamplifying means for amplifying each of said θ, h, h and h signals byindividual predetermined gain factors; first summing means for summingsaid amplified h, h and h signals and the amplified and high passfiltered θ signal; second high pass filter means for attenuating said θsignal below a predetermined frequency; second amplifying means foramplifying each of said θ, θ, h and h signals by individualpredetermined gain factors; second high pass filter means forattenuating said amplified θ below a predetermined frequency; secondsumming means for summing said amplified θ, h and h signals and theamplified and high pass filtered θ signal; and complementary filtermeans for outputting said first summing means signal at frequenciesbelow a predetermined frequency and outputting said second summing meanssignal at frequencies above said predetermined frequency, whereby theoutput of said complementary filter means is said produced elevatorcontrol signal.
 5. In an aircraft flight control system wherein innerloop damping signals are combined with outer loop command signals toproduce an elevator control signal, apparatus for producing an improvedinner loop damping signal, the apparatus comprising:means for providingsignals h and h representative of aircraft altitude and verticalacceleration, respectively; means for providing a signal θrepresentative of aircraft pitch rate; high pass filter means forattenuating said θ below a predetermined frequency ω₀ ; first circuitmeans for amplifying said h signal by a predetermined gain factor andpredeterminedly attenuating said amplified h signal above apredetermined frequency ω; second circuit means for amplifying theoutput of said high pass filter by a predetermined gain factor andpredeterminedly attenuating the resultant signal above said frequency ω;third circuit means for amplifying the output of said high pass filterby a predetermined gain factor and predeterminedly attenuating theresultant signal below said frequency ω; fourth circuit means foramplifying said h signal by a predetermined gain factor and attenuatingsaid amplified h signal below said frequency ω; combiner means forcombining the outputs from said first, second, third and fourth circuitmeans to thereby provide said inner loop damping signal.
 6. In anaircraft flight control system, apparatus for producing an elevatorcontrol signal, the apparatus comprising:means for providing signals θand θ representative of aircraft pitch and pitch rate, respectively;means for providing signal h, h and h representative of aircraftaltitude, altitude rate and vertical acceleration, respectively; firsthigh pass filtering means for attenuating said θ signal below apredetermined frequency; first amplifying means for amplifying each ofsaid θ, h, h and h signals by individual predetermined gain factors;first summing means for summing said amplified h, h and h signals andthe high pass filtered and amplified θ signal; second amplifying meansfor amplifying each of said θ, θ, h and h signals by individualpredetermined gain factors; second summing means for summing saidamplified θ and θ signals; washout filter means for high pass filteringthe output of said second summer means; third summing means for summingsaid amplified h and h signals and the output of said washout filtermeans; and complementary filter means for outputting said first summingmeans signal at frequencies below a predetermined frequency andoutputting said third summing means signal at frequencies above saidpredetermined frequency, whereby the output of said complementary filtermeans is said produced elevator control signal.
 7. In an aircraft flightcontrol system wherein inner loop damping signals are combined withouter loop command signals to produce an elevator control signal,apparatus for producing an improved inner loop damping signal, theapparatus comprising:means for providing signals h and h representativeof aircraft altitude, altitude rate and vertical acceleration,respectively; means for providing a signal θ representative of aircraftpitch rate; first circuit means for amplifying said θ signal by a firstpredetermined gain factor and attenuating the resultant signal below apredetermined frequency ω₁ ; second circuit means for amplifying said θsignal by a second predetermined gain factor and attenuating theresultant signal below a predetermined frequency ω₂ ; third circuitmeans for attenuating the output signal of the first circuit means abovea predetermined frequency ω; fourth circuit means for attenuating theoutput signal from the second circuit means above the predeterminedfrequency ω; fifth circuit means for amplifying said h signal by a thirdpredetermined gain factor and attenuating the resultant signal above thefrequency ω; sixth circuit means for amplifying said h signal by afourth predetermined gain factor and; combiner means for combining thesignals produced by the second third, fourth, fifth, and sixth circuitmeans to thereby produce said inner loop damping signal.